Glass substrates used for magnetic recording media such as hard disks must have a high degree of flatness and smoothness, as well as surface cleanliness. Glass substrates are therefore subjected to a step for surface polishing and a subsequent step for washing. The glass substrates for magnetic recording media which are obtained by such processes are also subjected to a texturing step as a manufacturing step for the magnetic recording media.
Efficient polishing of glass substrate surfaces is accomplished by using abrasive grains such as of cerium oxide as a polishing agent, but a problem is encountered when such abrasive grains remain on the glass surface after polishing. As a result, it is necessary to remove the adhered abrasive grains and glass shavings produced during the polishing step by washing prior to the texturing step. Because of increasing recording densities, even trace amounts of washing residue can be a source of defects when the magnetic recording medium is manufactured, and therefore a demand exists for more extensive washing techniques.
Glass currently used as substrates for magnetic recording media is largely divided into amorphous glass and crystallized glass, which is designed to exhibit improved strength by having minute crystal grains deposited in the glass through a crystallization process.
The use of organic acids or inorganic acids for washing of amorphous glass substrates is known in the prior art. For example, for manufacturing magnetic recording medium glass substrates composed of amorphous glass, there has been disclosed a technique of washing with oxalic acid after the texturing step (see Japanese Unexamined Patent Publication No. 2003-212603). Also disclosed is a technique of washing amorphous glass substrates with oxalic acid after chemical strengthening treatment (see Japanese Unexamined Patent Publication No. 2004-2163).
On the other hand, treatment of crystallized glass substrates with aqueous solutions of inorganic strong acids such as hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid or hydrochloric acid has been disclosed (see Japanese Unexamined Patent Publication No. 2000-311336), while treatment of glass substrates with strong alkalis has been disclosed as well (see Japanese Unexamined Patent Publication No. 2000-302482).
The present inventors have found that, although these washing methods are somewhat effective for removal of the polishing agent and the trace metal impurities or particles which adhere as contaminants during the processing steps, they create problems such as increased roughness of the crystallized glass substrate surfaces during their washing, and more frequent defects due to minute imperfections in the substrates.
In addition, it was found that when a magnetic recording medium is manufactured by forming a recording layer including a magnetic film on the substrate, the surface shape changes described above produce a phenomenon of impaired electromagnetic conversion properties, while it was demonstrated that washing of crystallized glass with a strong acid or strong alkali causes impairment of the quality of the magnetic recording medium.